Thermal retransfer printing involves forming an image (in reverse) on a retransfer intermediate sheet using one or more thermally transferable dyes. The image is then thermally transferred to a surface of an article by bringing the image into contact with the article surface and applying heat and possibly also pressure. Thermal transfer printing is particularly useful for printing onto articles that are not readily susceptible to being printed on directly, particularly three dimensional (3D) objects. Thermal retransfer printing by dye diffusion thermal transfer printing, using sublimation dyes, is disclosed, e.g., in WO 98/02315 and WO 02/096661. By using digital printing techniques to form the image on the retransfer intermediate sheet, high quality images, possibly of photographic quality, can be printed on 3D articles relatively conveniently and economically even in short runs. Indeed such objects can be personalised economically.
The image on the retransfer intermediate sheet can be formed by thermal transfer printing, e.g. as disclosed in WO 98/02315 and WO 02/096661. It is also possible to form the image on the retransfer intermediate sheet by inkjet printing using sublimation dyes. The media typically used for such retransfer printing comprises a paper substrate coated with layers which can absorb and then release the dyes printed in the inkjet process, e.g. as disclosed in EP 1102682. This type of material is very effective in transferring images to articles that are flat in two dimensions. However this material is not effective in transferring images to three dimensional objects. This is because the substrate used in the media is not flexible enough to form around the object without creasing and distorting. This results in uneven contact between the active surfaces, and prevents good transfer of the image onto the surface of the article to be decorated.
To overcome the problem of poor contact between active surfaces when attempting to retransfer printed images onto 3D articles, thermoformable substrates have been employed in place of a paper substrate. Typically the substrate used is amorphous polyethylene terephthalate, e.g. as disclosed in WO 01/96123 and WO 2004/022354. A problem often encountered in using such material is that the sublimation dyes typically used in this type of printing are very compatible with the substrate. Consequently, when carrying out the final retransfer step, the dyes can move into the substrate as well as transferring into the surface of the article being decorated, in a process called back diffusion. This means that not all the dye printed into the retransfer sheet is transferred to the final article, and limits the optical density achievable in the final image. As a result, images transferred lack contrast and are therefore perceived as being of low quality. To avoid back diffusion of dye into the substrate, barrier layers have been applied between the ink absorbing layer and the substrate. These barrier coatings are typically applied by sputtering of thin layers of metals such as aluminium. This adds substantially to the cost of the sheet assembly. In addition, such barrier layers tend not to be very flexible and can become crazed when the substrate is formed around the article to be decorated. The image that is subsequently transferred will reproduce this crazing through differential dye transfer, which once again detracts from the overall perceived quality of the final product.
U.S. Pat. No. 6,686,314 discloses a retransfer intermediate sheet comprising multiple layers on a substrate possibly of polyethylene terephthalate (PET), including an ink-absorbing layer that may contain porous silica gel and polymer such as polyvinyl alcohol.